diff --git a/Image.class.php b/Image.class.php
index 96c4df1..a6d386b 100644
--- a/Image.class.php
+++ b/Image.class.php
@@ -44,7 +44,14 @@ abstract class Image extends File
      */
     public static function getIsSubClass($class)
     {
-        return (get_parent_class($class) == __CLASS__);
+        $parent_class = get_parent_class($class);
+        if ($parent_class == __CLASS__) {
+            return true;
+        }
+        if (get_parent_class($parent_class)) {
+            return self::getIsSubClass($parent_class);
+        }
+        return false;
     }
 
     public static function getSizes()